1. Technical Field
This invention relates to magnetic read/write heads that employ TAMR (thermally assisted magnetic recording) using laser energy to heat magnetic media having high coercivity and high magnetic anisotropy. More particularly, it relates to methods that reduce reflections of the laser radiation back to the TAMR structures.
2. Description of the Related Art
Magnetic recording at area data densities of between 1 and 10 Tera-bits per in2 involves the development of new magnetic recording media, new magnetic recording heads and, most importantly, a new magnetic recording scheme that can delay the onset of the so-called “superparamagnetic” effect. This latter effect is the thermal instability of the extremely small regions of magnetic material on which information must be recorded, in order to achieve the required data densities. A way of circumventing this thermal instability is to use magnetic recording media with high magnetic anisotropy and high coercivity that can still be written upon by the increasingly small write heads required for producing the high data density. This way of addressing the problem produces two conflicting requirements:    1. The need for a stronger writing field that is necessitated by the highly anisotropic and coercive magnetic media.    2. The need for a smaller write head of sufficient definition to produce the high areal write densities, which write heads, disadvantageously, produce a smaller field gradient and broader field profile.
Satisfying these requirements simultaneously may be a limiting factor in the further development of the present magnetic recording scheme used in state of the art hard-disk-drives (HDD). If that is the case, further increases in recording area density may not be achievable within those schemes. One way of addressing these conflicting requirements is by the use of assisted recording methodologies, notably thermally assisted magnetic recording, or TAMR.
Prior art forms of assisted recording methodologies being applied to the elimination of the above problem share a common feature: transferring energy into the magnetic recording system through the use of physical methods that are not directly related to the magnetic field produced by the write head. If an assisted recording scheme can produce a medium-property profile to enable low-field writing localized at the write field area, then even a weak write field can produce high data density recording because of the multiplicative effect of the spatial gradients of both the medium property profile and the write field. These prior art assisted recording schemes either involve deep sub-micron localized heating by an optical beam or ultra-high frequency AC magnetic field generation.
The heating effect of TAMR works by raising the temperature of a small region of the magnetic medium to essentially its Curie temperature (Tc), at which temperature both its coercivity and anisotropy are significantly reduced and magnetic writing becomes easier to produce within that region.
In the following, we will address our attention to a particular implementation of TAMR, namely by the transfer of electromagnetic energy from an optical frequency laser diode (LD), through an optical waveguide (WG) to a small, sub-micron sized region of a magnetic medium, either directly, or, more typically through interaction of the magnetic medium with the near field of a surface plasmon in a plasmon generator (PG) excited by the laser radiation. The transferred electromagnetic energy then causes the temperature of the medium to increase locally to enable the write operation. However, in what follows, we will not eliminate the possibility that the electromagnetic radiation may be radiation that is other than optical frequency, in which case the conditions for suppression of reflections will have to be calculated using suitable boundary conditions at the interfaces.
As illustrated in schematic and prior art FIGS. 1(a) and 1(b), there is shown a front (x-z plane) view (1(a)) and a side (y-z plane) cross-sectional view (1(b)) displaying only the optical architecture of a TAMR device. It is understood that a magnetic write pole is located adjacent to this optical architecture (see (31) in FIG. 1(b)) so that a magnetic write field can be applied to the thermally heated spot on the recording medium.
Referring to the Cartesian coordinates in FIG. 1(a), the ABS plane (160) of the slider (20) is the x-y plane. The slider contains the read/write transducer elements ((30) in FIG. 1(b)) and its ABS surface is aerodynamically structured to fly over a rotating magnetic recording medium.
Returning to FIG. 1(a), there is shown the laser diode (300) affixed to a submount (40), with the combination being mounted on the back side surface (90) of the slider. The active portion of the laser diode is a Fabry-Perot-type resonant cavity (350). The bottom surface of the laser diode cavity, denoted its exit facet, couples to the top surface (52) of a waveguide (500). The waveguide passes through the slider, from the back end surface (150) of the slider, to its ABS (160) end, where the waveguide terminates. We note that a portion of the ABS end of the waveguide will generally also interface with and couple to an adjacent plasmon generator (see (32) in FIG. 1(b)) which absorbs electromagnetic energy from the waveguide mode and generates surface plasmons. The near-field of these plasmons can focus energy within a spot size on the recording medium that is not diffraction limited, as the laser radiation alone would be. Further details of the plasmon generator structure, other than its orientation relative to the waveguide, are not discussed in this disclosure but for the purposes of the description of this and subsequent figures, the plasmon generator may be considered as being in front of and immediately adjacent to the the ABS end of the waveguide, as will be indicated by (32) in FIG. 1 (b).
Referring to FIG. 1(b), there is shown a side cross-sectional view of the slider in FIG. 1(a), which is formed on a substrate (15) of AlTiC. There is also shown the side view of the laser diode (300), and the waveguide (500), which terminates in the ABS within the read/write transducer region (30). Within (30), there is also shown the relative positions of the writer (31), the reader (33) and the plasmon generator (32), the generator being situated between the writer (31) and the waveguide (500) and is adjacent to the ABS end of the waveguide (500). A portion of the rotating magnetic recording medium (70) is shown beneath the ABS. It is noted that the writer (31) essentially comprises an inductively driven magnetic write pole and it will be indicated simply as such a pole in FIG. 6, below.
The waveguide structure (500) illustrated in FIGS. 1(a) and 1(b) forms an external resonance cavity which is in addition to the laser diode's own resonance cavity (350) that supports the laser optical mode. In the illustrated configuration, the centerline (550) of the WG structure is normal to both its end surfaces: the backside end (52) which is coupled to the laser and the ABS end (54) that terminates at the ABS of the slider. Note that the centerline (550) is equally normal to both end surfaces of the slider.
Analysis of the TAMR action indicates that optical radiation from the laser is reflected at three interfacial surfaces:    (#1): the interface between the laser exit facet and waveguide at the back surface of the slider (the reflected radiation being shown as a U-shaped curved arrow (62));    (#2): the terminal surface of the WG (the reflected radiation shown as a U-shaped arrow (64)) at the ABS end of the slider (which can also include a plasmon generator) and;    (#3): at the surface of the recording medium (the reflected radiation shown as a U-shaped arrow (66)).
Following the propagation path, the laser light first passes the interface (#1) above, which is the interface between the emitting facet of the laser diode and the inlet end of the waveguide. At this interface, some laser light will be reflected back into the laser cavity due to the change in the refractive index. Laser light now couples with the waveguide mode and propagates towards the ABS end of the slider, whereupon it passes through the interface (#2) above, which is the ABS surface of the slider. Some light will also reflect back at this interface. In addition, light passing through the slider ABS across the gap between the slider ABS and the surface of the recording medium will also reflect back from the medium surface at interface (#3).
Each of these reflected radiation components can get back into the laser cavity and cause laser mode hopping, which are changes in the laser wavelengths and corresponding changes in the laser power transferred to these wavelengths. These unwanted variations cause changes in the temperature of the recording medium, introduce jitter into the recording bits and cause track-width changes to the recording process. In addition, the power ratio between the emitting side and the back side of the LD varies due to the reflected radiation which makes it very difficult, if not impossible monitor light intensity from the back side by means of an integrated photo-diode (PD) in order to achieve a feedback-controlled constant power output at the ABS end. Recent experiments in furtherance of this disclosure have confirmed the existence of back reflections for the TAMR optical architecture of FIGS. 1(a) and 1(b) by measuring the quantum shifts of the lasing wavelengths when increasing the current to the LD.
Although the following prior arts have discussed these issues: U.S. Pat. No. 8,274,867 (Mori et al.); U.S. Pat. No. 8,238,202 (Schreck et al.) and U.S. Patent Application 2012/0257490 (Zhou), neither the methods disclosed nor their results are the same as those to be described herein.